1.1.1 Glycoproteins and Biosynthesis1.1.1.1. Nitrogen-linked glycoproteins Nitrogen-linked glycoproteins were traditionally considered unique to eukaryotic systems. It is only more recently that their presence in bacteria and archaea has been recognized. N-glycans in eukaryotes share some common characteristics and have a common basic structure. They consist of common monosaccharide units. However, their structures in bacteria and archaea are more diverse and contain common and rare monosaccharide building blocks.1.1.1.1.1. N-linked glycoproteins in eukaryotic systems N-glycans in eukaryotic cells have a common core Man3GlcNAc2 linked to the Asparagine (Asn) residue in the Asn-X-Ser/Thr sequons of proteins (where proline) and can be classified into three types high mannose, complex and hybrid (Figure 1.1). They are displayed on many secreted and membrane-bound glycoproteins. Figure 1.1 Representative high-mannose N-glycans, complex and hybrid type. In Saccharomyces cerevisiae and vertebrates, they are initially synthesized on the cytoplasmic side of the endoplasmic reticulum (ER) membrane. , beginning with the transfer of N-acetylglucosamine phosphate (GlcNAc-P) from the nucleotide-activated sugar donor UDP-GlcNAc to the ER membrane-anchored molecule dolichol phosphate (Dol-P). The dolichol N-acetylglucosamine pyrophosphate (Dol-PP-GlcNAc) formed is then processed by glycosyltransferases. One GlcNAc residue and five mannose residues are then added from UDP-GlcNAc and GDP-Man, respectively, generating Man5GlcNAc2-PP-Dol. This sugar chain is flipped to the luminal side of the ER membrane and extended by transfer of four mannose residues from Dol-P-Man and three glucose residues from Dol-P-Glc. The 14-sugar pr...... middle of paper ......1–4Gal1–3diNAcBac–Neisseria gonorrhoeae pilin structure is assembled by PglB, PglA, and PglE on Und-P, flipped by PglF to the periplasm and transferred by PglO to the Ser residue of pilin [21]. In an alternative pathway, PglH adds one Glc residue rather than two Gal residues to diNAcBac. In OST-independent O-glycosylation, glycosyltransferases add monosaccharides to proteins in the cytoplasm and the resulting glycosylated proteins are transported to the outer membrane or secreted by the flagellum [22].1.1.1.2.3 Glycoproteins-related O in archaea The surface (S)-coat glycoproteins of archaea were found to be O-glycosylated. In Halobacterium salinarum, the cell envelope protein is modified with glucosylgalactose disaccharides and trisaccharides (uronic acid, glucose)-galatcotse at several sites (23). Little is currently known about the O-glycosylation pathway in archaea...